1. Field of the Invention
The invention relates to devices for processing plastic materials and, more particularly, to machines for injection moulding of thermoplastic polymers.
The invention may find a most effective use for injection moulding of variously-shaped items of small volumes of up to 40 cm.sup.3, e.g., for the manufacture of wine bottle stoppers, plug socket housings and various threaded sealing items.
2. Description of the Prior Art
There is known in the art a thermoplastic polymer injection molding machine (Zavgorodny V. K., "Mehanizatsiya i avtomatizatsiya pererabotki plasticheskih mass" /Mechanization and automation of plastics processing/, "Mashinostroeniye" Publishers, Moscow, 1970, p. 256-258) incorporating a continuously rotatable support carrying, on its periphery, injection cylinders and actuating hydraulic cylinders.
The machine incorporates a plasticator connected to the injection cylinders by means of a hollow shaft and heated radial channels in said support, a non-return valve being inserted in each of the channel outlet orifices. Injection dies are arranged on the perimeter of the support between the injection cylinders and the actuating hydraulic cylinders. Each injection cylinder carries a nozzle with an outlet orifice movable with respect to the injection cylinder and fitted with a rotary shut-off valve that prevents undesired discharge of polymer.
The productivity of such a machine is limited by the number of injection dies accommodated in the rotatable support which cannot be increased arbitrarily, as, in turn, it will lengthen the radial channels whose length is limited of the pressure development by the plasticator and the admissible dwell time in the distribution channels, with the effect that practical productivity is never higher than 20 mouldings per minute.
In addition, the injection cylinder nozzles in the machine are constantly outside the heated body of the rotatable support and are thus cooled excessively due to contact with the cold die air and the surrounding air, in particular, near the nozzle outlet orifices, this having an adverse effect on injection and, in consequence, upon the quality of moulded items.
And, finally, as the injection die is backed away from the injection cylinder, a portion of the molten material is forced out of the nozzle outlet orifice and may plug up or block the gate orifice.
Also known in the prior art is a machine for moulding items from a thermoplastic polymer incorporating at least one rotatable support with injection cylinders and actuating hydraulic cylinders set at its periphery. A plasticator is connected to the injection cylinders by means of a hollow shaft and heated radial channels in said support, each of the injection cylinder outlets carrying a non-return valve. Injection dies are placed on a closed-chain conveyor, bending round the support, between the injection cylinders and the actuating hydraulic cylinders, each injection cylinder incorporating a mobile nozzle with an outlet orifice, inserted into which is a rotary shut-off valve that prevents the undesired discharge of polymer.
This prior-art machine provides a considerably higher output as compared to the former device, as its productivity is in no way limited by the number of dies on the conveyor, this number being a function of the specified productivity target and the time necessary to cool moulded items outside the rotatable support. The substantial raise in productivity is due to a molten polymer being injected into the dies while they are on the rotatable support, whereas moulded items are cooled, ejected, etc., outside the rotatable support as the dies move with the conveyor.
However, the prior-art machine fails to attain a higher productivity due to disadvantages inherent in its design.
The productivity of the aforesaid known machine cannot be increased by increasing the rate of operation of injection cylinders as their service reliability decreases sharply due to the poor performance of non-return and rotary shut-off valves whose durabilities are low.
The productivity of the prior art machine is 50 to 120 items per minute as the nozzle is returned to the starting position after it co-operates with the injection die by overcoming the resistance the nozzle offers to the motion with the aid of the pressure exerted by the melt upon the nozzle end face, the value of the resistance being governed by the viscous friction of the molten material in the gap between the nozzle lateral surface and the internal side surface of the injection cylinder. A point to note is that viscous friction is an exponential function of the shear velocity of adjacent layers of melt, this rendering impossible a high back-out velocity of the nozzle to its starting position and the filling of the injection cylinder with a fresh portion of material even if the material is supplied at a high pressure.
Another disadvantage is that the machine fails to ensure the required quality of items, as the injection cylinder outlets are cooled excessively due to their location outside the heated support and their contact with the cold dies. The material then solidifies in the outlet, is forced inside the dies during the injection that follows and thus may be responsible for the discarding of the item.
As the injection cylinders in the said machine remain outside the heated support for a substantial period of time, they tend to cool and thus cool the portion of the material they hold, this prohibiting the operation of the machine on thermoplastic materials that require accurate maintenance of their temperatures within a narrow range before injection into dies.
And, finally, the great number of non-return valves and rotary shut-off valves complicates the machine and increases its cost.